This article deals with the 'formats' in which sound exists, is transmitted and distributed in the context of live, recording and consumer settings. First it is important to note that sound is analogue, it consists of waves with specific speed, wavelength and frequency. In order for sound to be transmitted from the 'real world' to a system, the energy must first be converted to electrical signals with a transceiver, typically a microphone or 'mic'. Each mic usually occupies one track. For the purposes of this article a channel will be referred to as a sum of one or more tracks into a single signal.
Sound exists in several different formats, containing different amounts of channels.
The simplest form of analogue sound is a single channel of signal consisting of an electrical representation of sound energy. A single channel of sound is referred to as a monophonic signal. Radio, in particular AM radio, is distributed using one channel which is converted back to sound energy by the user's radio. Older televisions and radios may only have monophonic capability because they only have one speaker or a one channel receiver.
For live work, although a two channel system covering the left and right sides of the room can be used, the same signal is often sent to both speakers. This helps to reduce phase interactions between cabinets while increasing the coverage and spread of the speakers.
Stereo makes use of two channels of concurrent audio and currently is a popular format of distribution for music, television and FM radio. Most Hi-Fi and television systems have two speakers, each of which carry a channel. Stereo became popular because it gave producers greater room to layer sounds and create other artistic effects.
Various miking techniques make use of two microphones to capture more of the dynamic sound of the instrument they are recording, for example a technique for recording an acoustic guitar may use two microphones to record different areas of the guitar. Once the signals have been recorded they can be placed towards the left or right 'side' of the mix.
A further use of stereo is give the listener a perception of the source of the sound sitting in front of them and enables them pick out each sound. For example, a rock track may be recorded with tracks panned in such away to allow the user to visualise the rhythm guitarist on the left of the stage while the lead may be on the right side. It is unusual to find a track of sound completely panned to a side because as aforementioned, some users still use mono equipment and depending on their setup may miss out on some of the sound.
In some cases, the audio signal on the left may cause phase interactions to the audio signal on the right channel. These interactions can boost and cut certain waves as they interact with each other, for example two waves which were exactly 180 degrees out of phase may completely cancel each other, thus leaving no sound. In stereo systems these interactions can cause comb filtering which is usually an undesired effect.
Live environments can make use of stereo panning just as they can in recorded media but over complicated miking techniques are often avoided for the sake of simplicity. Hard Panning in Live Sound Reinforcement is often frowned upon. Whilst it is normal for certain tracks or channels to be panned slightly left or right hard panning is not recommended. This is because a member of the audience on the left of the room will not hear a sound panned to the opposite side. This is often why CD playback is sent to 2 mono channels on a mixing console rather than a stereo channel. Older music (e.g., 1960's and 1970's) made use of hard panning and thus in a scenario such as a disco or even background music during a show the full track is impossible to hear at the sides of the room. More advanced mixing consoles may have a stereo width control on the stereo inputs, Some DSP units such as digital equalisers and crossovers also have this function. This control can normally be adjusted from MONO to STEREO and anywhere between.
Finally, phase interactions can be mathematically calculated to created wide stereo fields in both vertical and horizontal planes, this 'quasi-stereo' can be used in games and television to make the listener feel the sound is 'around' them rather than just coming from the speakers.